In an ultrasonic generator known from the U.S. Pat. No. 3,596,206, an oscillator with a transistor of a self-oscillating design is known which is disposed in the driver circuit of a piezoelectric converter in combination with an amplifier and an impedance converter, both in series circuit array. For such an application, the transistor is connected in series to the primary winding of the impedance converter, i.e. a transformer, so as to constitute a modified Meissner circuit, and is set to its resonance working point by means of a voltage divider. The voltage divider includes a connection to a reaction coil which is inductively coupled to the primary winding of the transformer. In this way the transistor is given a frequency which equals the series resonance frequency of the piezoelectric converter in series with the secondary winding of the transformer. The driver circuit of the converter, or its series transformer, respectively, are moreover supplied from a power-pack through a rectifier connected to the primary side of the transformer and through a smoothing capacitor parallel thereto. The reaction coil of the transformer and the transistor are connected to each other through a series circuit including a capacitor and a resistor. That series circuit serves to match the reaction, which prevails between the piezoelectric converter and the transistor and which may be deactivated arbitrarily by means of an interrupter so as to achieve protection of the converter.
The French Patent FR-A-81 09 330 also discloses an ultrasonic generator designed to include a piezoelectric converter as well, wherein the circuit array includes not only a transistor providing a comparably self-oscillating oscillator but also a constant-current source provided with two further transistors and several resistors. In that circuit the potentiometer serves to trim the limiting current which is supplied to the converter when power acceptance is missing.
In the known ultrasonic generators with a piezoelectric converter, the self-oscillating design of the oscillator creates a direct dependence of the conditions of converter operation on the tolerances of the circuit elements directly connected thereto. Variations in the mutually matched tolerances of the components, which are mostly unavoidable in mass production based on devices obtained from different suppliers, create critical interfering effects on the oscillator's characteristics of oscillation and thus on the power output of the piezoelectric converter, too. The elimination of such interfering effects is possible only with a higher expenditure in terms of devices and costs.
The German Patent DE-A-36 41 058 discloses an ultrasonic generator with a magnetostrictive converter whose energizing coil is disposed in the collector circuit of a transistor. The emitter circuit of the transistor is connected through a resistor disposed to detect the current flow through the energizing coil and through a voltage evaluation circuit to control means causing the supply of the energizing pulses to the base of the transistor. The voltage evaluation circuit supplies a digital signal in response to the voltage drop at the resistor and is constituted either by a series circuit including a voltage frequency divider and a counter, or by a series circuit including a DC voltage suppression circuit, a low-pass circuit, and an A/D converter. The output of the control means is connected to a counter which emits pulses at a pulse rate determined by the signal supplied by the voltage evaluation circuit. The transmission of these pulses to the base of the transistor is determined by a timed switching regulator which is controlled by the control means. The control means is also connected to a potentiometer so as to provide for variation of the power output preset for the magnetostrictive converter.
In these known ultrasonic generators with a magnetostrictive converter the control means is only provided for setting of the pulse rate of the energizing pulses to the resonance working point of the converter. In these devices, during a primary trimming phase of the circuit array, pulses of a respectively predetermined number of pulse rates are supplied to the energizing coil in respectively successive smaller pulse rate ranges. Moreover, an uncomplex error indicator is provided as a monitor of each of the pulse rate ranges of the energizing pulse which are thus invariably set at individual trimming levels by the start of operation, at which the magnetostrictive converter operates at a resonance frequency, that monitor being under control of the control means such that any variation of that resonance frequency from the invariably set pulse rate range furnishes the mere indication of an error that has occurred, without correcting the actual source of the error at the same time.
The present invention refers to the problem of designing an ultrasonic generator with a piezoelectric converter in a way that the power output of the converter is less critically dependent on tolerances of the connected devices of the circuit array and that also an optimum possibility of correction of that power output is achieved when the set resonance working point of the converter should vary during operation of the ultrasonic generator.